Using FTDI F232RL for USB to serial interface at the output of a UPS. Chip keeps freezing when connected to PC application. It works again after it is taken taken out. Application is written by us, and we can see that the chip is hanging at that point. If I use another PC to emulate the UPS, then it works fine. Even when I test it between two PCs to transfer info, it works fine even if keep connecting and disconnecting the serial port side every second. It works in a very stable way if I remove the USB and put it back many times.

We can connect the UPS output to other devices that accept serial port data - remote display / SNMP agent hardware etc. All of them run fine. UPS is ours. We don't find any noticeable spikes at the serial port side. Serial port of the UPS, is also opto isolated from the other part.

Problem happens only when the card serial port side (just a small pcb , with f232 and max232 as per their pdfs) is connected to sme other serial port. If I make a test device with a pic controller + max232 serial port powered from a linear regulator sourcing power from a 9v transformer + rectifier, and use that device to drive my serial to USB, freeze-up happens ie: f232r hangs if I keep connecting and removing the ground pin of serial port. Else, it is stable.

FTDI guy says the problem is with noise. We have done more than what they recommended in their documents. Proper decoupling, ferrite beads even in Rx/Tx/Line of serial port etc. Tried bigger caps at the power supply source side of PCB (USB bus powered) - even tantalum ones. Error handling is done in PC app, including pipe reset count, timeouts etc.

I have seen some posts talking about F232R freezeup when relay is activated/ motor is running etc.But these posts were not with useful resolutions.

Any suggestions to deal with the freeze-up? FTDI guys are not being helpful.


Regards, Raj